Discipline
It's so easy to fall out of good habits and into bad ones. This is one of the most frustrating parts of human existence, and something we all struggle with.
This is yet another example of the universally corrosive force of entropy and here, just like everywhere else, we mustn't give it any quarter.
One lazy day won't ruin everything, but it is the only way to have two lazy days, which can quickly become three, and so on and so forth.
One lazy day (and remember, there is a difference between laziness and conscious rest) destroys your momentum, making it even harder to stay on whichever straight and narrow path you have chosen for yourself. Its consequences insidiously go far beyond the day in question, like a destructive shockwave, or a ripple across a lake.
The only way out is through. Push! Push! Push!